随机读写存储器可分为：（）。
A.静态和动态存储器
B.静态和只读存储器
C.动态和只读存储器
D.可擦除和动态存储器
参考答案：A
— Did you find the missing couple in the mountain yesterday?
— No, but we ____ to get in touch with them ever since. [ ]
A. have tried
B. have been trying
C. had tried
D. had been trying
（）膨胀机机前温度可以使气体通过膨胀机前后的温差增大。
